Lubrication system for a drilling apparatus

ABSTRACT

The present invention provides a lubrication system for a drilling apparatus. The drilling apparatus includes a drill string and a drill bit, and it is driven by a working fluid circulated through a working fluid circuit. The lubrication system uses a reservoir of a lubricating fluid for supply to a drilling apparatus during a drilling operation; and a pump for supplying lubricating fluid from said reservoir to the drilling apparatus during the drilling operation. The pump is controlled to supply lubricating fluid from the reservoir at a determined rate to a lubrication circuit formed by the working fluid circuit of the drilling apparatus.

FIELD OF THE INVENTION

This invention relates to a lubrication system for a drilling apparatus.

BACKGROUND TO THE INVENTION

Drilling apparatus are used in exploitation of mineral resources.Reverse circulation drilling apparatus are used in exploration andnormal circulation drilling apparatus are used for mining andexploration applications. Drilling of blastholes and geological samplingare just two applications for such drilling apparatus which is typicallysubjected to very severe operating conditions. The drilling apparatusmay be rotary or percussive.

A percussive drilling apparatus, such as a hammer, comprises a drillstring having, at its working end, a drill bit mounted in a drive sub.The drilling apparatus is powered by a working fluid, typicallycompressed air, the pressure of which is used to drive a drill bit,whilst the drill string is rotating, to impact and drill a formation. Tothis end, such drilling apparatus has a working fluid circulation systemfor directing working fluid from the compressor through the drillingapparatus to the drill bit from which spent working fluid is exhausted.Such working fluid circulation system or circuit may be complex. Drivingaction is caused by fluctuations in working fluid pressure which drive apiston, in a working chamber forming part of the working fluid circuit,to impact the drill bit and generate drilling action and productcuttings which are recovered, in a flushing and/or exhaust flow, asrequired. Reciprocation of the piston generates considerable frictionand potential for wear. Other parts of the drilling apparatus are alsolikely subject to wear.

To date, despite the operating conditions to which a drilling apparatusis subjected, lubrication practice has typically involved cracking, oropening, of the drill string—at an operator selected point—and pouringof lubricant into the drill string. This lubricant then finds its waythrough the working fluid circulation system, which typically forms thelubricant circuit for drilling in accordance with this lubricationpractice, and to the working end of the drilling apparatus. Suchlubrication practice, while surprisingly effective in the short term, isunlikely to allow for the correct quantities of, or rates of supply of,lubricant to be applied to the drilling apparatus.

In percussive drilling, wear damage and component failure, includingdrill bit failure through shanking of a drill bit, will result ifinsufficient lubricant is supplied. Even if failure is avoided,insufficient lubrication may prevent sealing within the working fluidcircuit, for example between piston and working chamber walls as definedby wear sleeve or inner cylinder if an inner cylinder accommodates thepiston. In the case of sub-optimal sealing, drilling efficiency isreduced.

If too much lubricant is supplied, drilling apparatus operation may beotherwise affected and there is some cost impact as well. However,subject to avoidance of “dieseling” where lubricating fluid in excessivequantity ignites, supply of too much lubricant is generally less of aproblem than supply of too little.

Current drilling apparatus lubrication practice involving drill string“cracking” therefore interferes with drilling operations whilstpresenting a hazard to the operator. Lubrication, by definition, isrequired whilst a drilling apparatus is operating. This means that theoperator is working whilst machinery is in operation and this presentsobvious hazard.

Current practice in drilling presents both disadvantages of potentiallyinsufficient lubrication of the drilling apparatus and sub-optimaldrilling efficiency whilst hazard exists for the operator applying thelubricant.

U.S. Pat. No. 4,508,183 describes a method and apparatus for providing aflow of lubricating fluid from a surface mounted lubricating fluidsource through a rotatable drill pipe to rotary cutting cones of arotary drill bit for drilling in an underground formation, includingpump means for providing a flow of lubricating fluid from the fluidsource to an end of the drill pipe. Drill pipe conduit means extendingalong the length of the drill pipe is provided for receiving the flow oflubricating fluid and for directing the lubricating fluid flow along thedrill pipe to the drill bit. The drill pipe conduit means is a galleyline included within the wall of the drill pipe of each section of drillpipe. The galley line extends from the upper end to the lower end ofeach drill pipe and may be formed by casting or machining a keyway orslot along most of the drill pipe section. A suitably sized elongatedcover member may then be fixed welded in place along the surface of thedrill pipe wall to fill in a portion of the slot and to turn the slotinto an enclosed conduit. Distribution conduit means at the drill bitextends from the drill pipe conduit means to the cutting cones andreceives the flow of lubricating fluid from the drill pipe conduit meansfor directing the lubricating fluid flow to the cutting cones forlubrication.

US 2011/0031018 describes a horizontal direction drilling systemcomprising a power pack coupled to a source of compressed air and awater reservoir and forms a mixture of compressed air, water and oil.The horizontal direction drilling system further comprises a steerablehorizontal drill. The steerable horizontal drill includes an air poweredreciprocating hammer, and a drill head. The steerable horizontal drillreceives the mixture to power the reciprocating hammer. The drill bitincludes a drill face and the mixture exits the steerable horizontaldrill through the drill face. The amount of lubricant and water can berespectively controlled to control the proportions in the mixturepowering the reciprocating hammer. Horizontal directional drilling (HDD)is commonly used for routing utilities through pipes, ducts and cablesthrough underground impediments such as rock structures. To that end thehammer includes a sonde to identify position and orientation of thedrill so that it can be steered.

It is an object of the present invention to provide a lubrication systemto improve the lubrication and drilling efficiency of earth drillingapparatus.

SUMMARY OF THE INVENTION

With this object in view, the present invention provides a lubricationsystem for an earth drilling apparatus including a drill string and adrill bit, said drilling apparatus being driven by a working fluidcirculated through a working fluid circuit comprising:

a reservoir of a lubricating fluid for supply to a drilling apparatusduring a drilling operation; anda pump for supplying lubricating fluid from said reservoir to saiddrilling apparatus during said drilling operation,wherein said pump is controlled to supply lubricating fluid from thereservoir at a determined rate to a lubrication circuit formed by saidworking fluid circuit of said drilling apparatus.

The lubrication system may be used in percussion drilling apparatus orrotary drilling apparatus.

In another aspect of the invention, there is provided a method ofdrilling a formation with an earth drilling apparatus having a drillstring comprising a drill bit, said drilling apparatus being driven by aworking fluid circulated through a working fluid circuit of saiddrilling apparatus comprising lubricating the drilling apparatus duringa drilling operation wherein said drilling apparatus comprises alubrication system having a reservoir of a lubricating fluid forlubricating the drilling apparatus; and a pump for supplying lubricatingfluid from said reservoir to said drilling apparatus during saiddrilling operation wherein said pump is controlled to supply lubricatingfluid from the reservoir at a determined rate to a lubrication circuitformed by said working fluid circuit of said drilling apparatus.

The drilling apparatus is conveniently operated from a drilling rig, thedrilling rig comprising equipment items, such as air compressor,hoisting equipment, hydraulics and other accessories for operating thedrilling apparatus. The drilling rig is surface located and the pistonpump and lubricating fluid reservoir of the lubrication system areconveniently and advantageously surface located also, advantageously, ona deck of the drilling rig. This allows easier servicing of the pump, ifrequired, and filling of the lubricating fluid reservoir when required.

A conventional working fluid for drilling apparatus is compressed air.In such case, live compressed air may be supplied through a workingfluid circuit extending from an air compressor, also convenientlylocated on any drilling rig, through a check valve, and choke ifprovided for control of exhaust fluid flow rate for flushing cuttings,and through the drill string to its working end located at the base of ahole being drilled. It is not envisaged that water would be commonlyused as working fluid for the drilling apparatus.

The working fluid circuit must provide working fluid to a workingchamber located within the drilling apparatus to operate a reciprocatingpiston within the drill string near its working end. As the pistonreciprocates in its working chamber in response to cyclic pressurevariations in volumes above and below the piston in the working chamber,it impacts the drill bit at high frequency to fracture material intocuttings. The working fluid pressurises a drive volume of the workingchamber when driving the piston, in a drive stroke, to impact the drillbit. Working fluid is then exhausted from the working fluid circuitthrough or past the drill bit to flush cuttings away from the cuttingface of the drill bit in a flushing flow. Following impact of piston ondrill bit, working fluid pressurises a return volume of the workingchamber to drive the piston, in a return stroke, back to the top of theworking chamber in preparation for a further drive stroke. Working fluidis also exhausted through or past the drill bit during the returnstroke.

The system and method of the invention involve entraining a determinedamount of lubricant within the working fluid. This may atomise thelubricant to provide more beneficial lubricating effects. The workingfluid circuit, as for example above described, therefore forms thelubrication circuit for the drilling apparatus. The above description ofthe working fluid circuit is generalised and other working fluid circuitconfigurations, though including the same essential elements, may beadopted by those skilled in the art. The system and method of theinvention do not require fabrication of drilling apparatus components toinclude conduits solely for delivering lubricant to the drillingapparatus. The system and method are therefore conveniently, and costeffectively, applied to conventional drilling apparatus.

The lubricating fluid includes a lubricant used to lubricate thedrilling apparatus and especially moving parts thereof. Such lubricantsmay be described as rock oils and are selected for the drillingapplication.

Though many components of a drilling apparatus require lubrication, asimportant examples, lubricant must be supplied to a drill bit/drive subassembly connected to the drill string to avoid drill bit/drive subfailure as well as to the piston for driving the drill bit. Typically,the drill bit and drive sub are provided with respective andcomplementary spline portions formed about their perimeter. A pluralityof spline portions are provided on both drill bit outer or shank surfaceand drive sub inner surface which interlock to form the drill bit/drivesub assembly. The assembly allows the drill bit to slide (though notrotate) relative to the drive sub. Lubrication at the spline portions isimportant to prevent seizing and other forms of failure collectivelyknown as “shanking”. The lubricant pump therefore delivers lubricant atsufficient rate to form a film on these complementary drill bit anddrive sub spline portion surfaces. Spaces between the interlockingspline portions act as channels for working fluid to pass through to thecutting end of the drill bit so also forming part of the working fluidcircuit and lubrication circuit in this type of drilling apparatus.

Lubricating fluid is also used to create an oil seal between a pistonwhere used for driving the drill bit and the wall of the piston workingchamber. The working chamber wall may be formed by the inner wall of awear sleeve or the inner wall of an inner cylinder located within thewear sleeve. This oil seal is required to significantly reduce or avoidworking fluid leakage between drive and return volumes of the workingchamber. An effective oil seal increases pressurisation of drive andreturn volumes of the working chamber on drive and return strokesrespectively and optimises drilling efficiency as measured bypenetration rate.

The lubricating fluid may also include foams, detergents and otheragents, particularly surface active agents, for assisting drillingoperations and is used, in this specification, with that sense. Foams,in particular, are often used to assist air drilling operations byassisting with cuttings removal in flushing air flows. Such foams may becorrosive.

The lubrication system may be used to supply a plurality of lubricatingfluids in which case it may conveniently include a plurality ofreservoirs, each reservoir corresponding with a particular lubricatingfluid. For example, one reservoir could contain foam and anotherreservoir, a lubricant.

The determined rate of supply of lubricating fluid to the drillingapparatus may depend on one or more of a plurality of factors includingdrill string or hammer diameter, formation type (which influencesdrilling apparatus operating conditions) and drilling apparatus workingfluid pressure, typically compressed air pressure. Other factors mayalso be relevant.

The pump is conveniently, and advantageously, a hydraulically operatedpump. Hydraulic fluids, such as hydraulic oils, may be used to drive thepump using a timer or control valve device for controlling hydraulicfluid pressure within the pump (and, consequently, delivery pressure ofthe lubricating fluid to the drilling apparatus), cycle time andlubricating fluid supply frequency and rate. Pressure regulators may beincluded in the lubrication system to reduce hydraulic fluid pressuresto suitable levels for operation of the pump, it being understood thathydraulic fluids used for drilling operations, and sourced fromhydraulic fluid pumps conveniently located on drill rigs, are typicallyat high pressure, higher than required to drive the pump of thepresently described lubrication system.

The pump is advantageously a piston pump which draws a predeterminedquantity of lubricating fluid from the lubricating fluid reservoir on anintake stroke of the pump and expels it on an exhaust or delivery stroketo the drilling apparatus. The stroke refers to the movement of a pistonwithin a working chamber of the pump. The working chamber may comprisetwo variable volume chambers, one on the delivery side of the pump andthe other on the pump side. The pump side chamber is pressurized by ahydraulic fluid for operating the pump during a delivery stroke. Thestroke or length of travel of the piston may also be controlled tocontrol rate of intake of lubricating fluid into the hydraulic sidechamber and supply of lubricating fluid to the drilling apparatus.Alternatively, a diaphragm pump could be used.

The pump is connected, through a lubricating fluid system, both to thelubricating fluid reservoir and to the drilling apparatus. Return oflubricating fluid to the reservoir during a pump delivery stroke or tothe drilling apparatus on an intake stroke should be prevented. To thisend, the lubricating fluid system may be provided with check ornon-return valve(s) in supply and delivery lines to prevent return oflubricating fluid to the reservoir when supply to the drilling apparatusis intended or to the drilling apparatus, in uncontrolled manner, whensupply to the pump is required in an intake stroke.

BRIEF DESCRIPTION OF THE DRAWINGS

The lubrication system for a drilling apparatus of the present inventionmay be more fully understood from the following description of apreferred non-limiting embodiment made with reference to theaccompanying drawings in which:

FIG. 1 is a perspective view of the lubrication system for drillingapparatus of one embodiment of the present invention.

FIG. 2 is a side view of the lubrication system of FIG. 1.

FIG. 3 is a top view of the lubrication system of FIGS. 1 and 2.

FIG. 4 is an end view of the lubrication system of FIGS. 1 to 3.

FIG. 5 is a section view of the pump included within the lubricationsystem of FIGS. 1 to 5.

FIG. 6 is an exploded view of the pump included within the lubricationsystem of FIGS. 1 to 5.

FIG. 7 is a side section view of a drilling apparatus lubricated usingthe lubrication system of one embodiment of the present invention.

FIG. 8 a is a section view of the drilling apparatus along section lineA-A of FIG. 7 and showing the drill bit/drive sub assembly.

FIG. 8 b is a enlarged view of the drilling apparatus showing detail Bof FIG. 8 a.

FIG. 9 is a side view of the drill bit included within the drillingapparatus as illustrated in FIGS. 7 and 8.

FIG. 10 is a perspective view of the drive sub included within thedrilling apparatus as illustrated in FIGS. 7 and 8.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIGS. 1 to 10, there is shown a lubrication system 10 for adrilling apparatus. The drilling apparatus is shown in FIGS. 7 to 10which illustrate a percussive drilling apparatus in the form of a normalcirculation down the hole (DTH) hammer 100 including a drill bit 130.Drill bit 130 has, viewed rearwardly along its length, a head portion130 a and a shank 130 c with an intermediate portion 130 b and acylindrical portion 130 d. Cylindrical portion 130 d is relatively shortin comparison to the length of the intermediate portion 130 b of shank130 c. Head portion 130 a includes a cutting face 311. The surface ofintermediate portion 130 b of drill bit 130 is formed with a pluralityof spline portions 131 correspondent with and, and interlocked with,respective and complementary driving spline portions 128 of a drive sub120 forming a further part of DTH hammer 100. The drill bit 130 canslide relative to the drive sub 120 but not rotate relative to it.Spaces 136 are left between the spline portions 128 and 131.

DTH hammer 100 includes a wear sleeve 110 which, near the working end ofDTH hammer 100, is located a working chamber 144 provided with a piston150. Live compressed air is supplied from a compressor (not shown) ofthe DTH hammer 100 and through a compressed air (working fluid) circuitdelivering compressed air to 1) operate the piston 150 and 2) flushcuttings away from the cutting face 311 of drill bit 130 with assistanceof a foam. This compressed air is the working fluid for DTH hammer 100.Compressed air flows through the compressed air circuit extending fromthe compressor 195, check valve 155 in top sub 152, through centralpassageway 153 of top sub 152 and through a further portion of thecompressed air circuit to working chamber 144 to operate the piston 150.The compressed air circuit extending downstream of piston 150 isdescribed below.

Piston 150 reciprocates in the working chamber 144 in response to cyclicvariations in the pressure of drive and return volumes 144 a and 144 bof the working chamber 144. On drive strokes, piston 150 impacts thedrill bit 130 at high frequency. As drill bit head portion 130 a,provided with round headed cutter elements or buttons 135 of tungstencarbide or other suitable material, impacts the bottom of a hole duringa drilling operation, cuttings are formed and returned to the surfaceentrained in exhaust air from the compressed air circuit. At leastportion of this exhaust air is delivered from working chamber 144depending on choke arrangements (if used).

Compressed air pressurises drive volume 144 a of the working chamber 144when driving the piston 150, in a drive stroke, to impact the drill bit130. Compressed air is then exhausted from the drive volume 144 athrough a downstream portion of the compressed air circuit extendingfrom drive volume 144 a and through the spaces 136 between the splineportions 128 and 131.to finally exit past the drill bit 130.

Following impact of piston 150 on drill bit 130, compressed airpressurises return volume 144 b of the working chamber 144 to drive thepiston 150, in a return stroke, back to the top of the working chamber144 in preparation for a further drive stroke. Compressed air is alsoexhausted through the above described downstream portion of thecompressed air circuit during the return stroke.

DTH hammer 100 requires lubrication of its moving parts by a lubricatingfluid such as a rock oil. Lubrication at the spline portions 128 and 131of the drill bit/drive sub assembly 125, and the spaces 136 betweenthem, is particularly important to prevent seizing and other forms offailure collectively known as “shanking”. Lubricant is required insufficient quantity to form an effective lubricating film on thesecomplementary drill bit and drive sub spline portion 128, 131 surfaces.Attempting to deliver lubricant only by the “cracking” method, andpouring lubricant into the drill string of DTH hammer 100 cannot ensurethat this requirement is achieved.

Lubricating fluid is also required to form an oil seal between the innerwall of wear sleeve 110 and piston 150, that is, an oil film on wallsurfaces defining the working chamber 144 of the DTH hammer 100. Thisoil seal is required to significantly reduce or avoid compressed airleakage between drive and return volumes 144 a, 144 b of the workingchamber 144. An effective oil seal increases pressurisation of drive andreturn volumes 144 a, 144 b of the working chamber 144 on drive andreturn strokes respectively and optimises drilling efficiency asmeasured by penetration rate.

Check valve 155, regulating compressed air flow to the drill bit/drivesub assembly 125, is another component lubricated by the lubricationsystem.

Lubrication system 10 ensures that the objectives of lubrication andsealing to promote drilling efficiency of are effectively achieved.

Referring now to FIGS. 1 to 6, lubrication system 10 includes areservoir 20 of lubricating fluid for supply to DTH hammer 100 during adrilling operation. Reservoir 20 is a plastic tank with 30 litrecapacity (in the example shown; reservoir 20 may have any desiredcapacity dependent on the lubricating fluid to be supplied) and it maycontain, as lubricating fluid, a foam or detergent for assisting airdrilling operations. A suitable plastic, such as HDPE, is selected forcorrosion resistance as foams and detergents are often corrosive. It mayalso, as here, include a lubricant being a rock oil of requiredspecification for use in lubrication of the moving parts of the drillingapparatus and notably the drill bit/drive sub assembly and the piston150 for driving drill bit 130 during a drilling operation. Tank 20 isprovided with a plastic cap 22 which can be removed to enable top up oflubricating fluid as required.

Lubrication system 10 includes a pump 30 for automatically supplying thelubricating fluid from tank 20 to the DTH hammer 100 during a drillingoperation. Pump 30 is controlled to supply lubricating fluid fromplastic tank 20 at a determined rate to the lubrication circuit of theDTH hammer 100. It may be understood, from the above description,requiring that lubricant be supplied to the working chamber 144 anddrill bit/drive sub assembly 125, that this lubrication circuitcorresponds with the compressed air circuit used for operating the DTHhammer 100.

The reservoir 20 and pump 30 are located within a housing 10 a of thelubrication system 10 surface located on a drilling deck 290 of adrilling rig (not shown) used for operating DTH hammer 100. Thelubrication system housing 10 a is bolted to this deck by bolts 83 alocated at mounting plates 83 of the lubrication system 10. The housing10 a may readily be disconnected, removed for maintenance and replacedby a further housing 10 a, if necessary.

Pump 30 is hydraulically operated using a supply of hydraulic oil atpredetermined pressure to drive a double sided piston 34 having twopiston heads 34 a and 34 d connected by a rod 34 b and fixed with a nut49. The hydraulic oil may be supplied from the hydraulic equipment ofthe drilling rig and may be different to the rock oil used forlubrication of DTH hammer 100.

As seen most conveniently in FIGS. 5 and 6, pump 30 has a cylindricalhousing 31 comprised of two cylindrical housing portions 31 a and 31 beach threadably connected to a gland centre 34 c spacing portions 31 aand 31 b. Gland centre 34 c includes a bush 48 to provide a seal forpiston rod 34 b at the gland centre 34 c. Gland centre 34 c also acts asa limit to travel or stroke of piston head 34 a.

The cylindrical housing 31 of pump 30 contains a working chamber. On itspump side and within cylindrical housing portion 31 b, is contained avariable volume chamber 32 with a lubricating fluid port 33 throughwhich lubricating fluid is supplied to and from the pump 30 toward DTHhammer 100. A breather 38 is also provided for chamber 32. On thehydraulic side and within cylindrical housing portion 31 a is containeda variable volume chamber 35 with a hydraulic oil inlet 36 formed inpump end gland 46 for supplying hydraulic oil to drive pump 30 and ahydraulic oil outlet 37 for discharging hydraulic oil from pump 30following a lubricating fluid supply event. Chambers 32 and 35 are shownseparated by reciprocating piston 34 and the chambers 32 and 35 vary involume depending on piston 34 position within cylindrical housing 31.

Piston 34 moves in one direction (to the left in FIG. 5) to deliverlubricating fluid from pump 30 to DTH hammer 100 when required on alubricating fluid delivery stroke. Then, when lubricating fluid has beendelivered from pump 30 to the drilling apparatus and hydraulic oil—usedin driving the piston 34—has been vented from pump 30 through oil outlet37 and line 39, the piston 34 moves in the opposite direction (to theright in FIG. 5) creating suction in chamber 32 and enabling lubricatingfluid to be drawn from tank 20 into chamber 32 on an intake stroke.Hydraulic oil is not supplied to chamber 35 during the intake stroke ofpiston 34.

When delivery of a further quantity of lubricating fluid to the drillingapparatus is required, hydraulic oil is supplied from the hydraulic oilsupply through supply hose 42 and onward through supply duct 44 tochamber 35. This supply of hydraulic oil causes piston 34 to move, in adelivery stroke, to drive lubricating fluid from chamber 32 towards DTHhammer 100. Intake and delivery stroke timing are separated bycontrolled time duration, regardless of any lag before piston 34 startsto move to draw lubricating fluid into chamber 32. This controlled timeduration, necessary to achieve determined lubricant supply rate, islikely to be in the order of seconds. However, longer duration ispossible dependent on lubricating fluid requirements of the DTH hammer100 and drilling conditions which influence these requirements. Higherrated rock oils may be required as lubricating fluids as drillingconditions become more demanding. For example, drilling of a harder rockformation is likely to require higher rated rock oils at higher ratesthan when drilling of a soft rock formation.

Pump 30 operation has cycle time comprised of time for intake oflubricating fluid into chamber 32; time for building hydraulic oilpressure in chamber 35 for driving piston 34 to deliver lubricatingfluid through port 33 towards the drilling apparatus; and time fordelivering lubricating fluid through port 33 in a delivery stroke. Thecycle time is controlled using a three way solenoid actuated or ON/OFFhydraulic oil control valve 40 which is connected to hydraulic oilsupply hose 42 (rated to 400 Bar) and both to the hydraulic oil inlet 36and hydraulic oil outlet 37 of pump 30. Hydraulic oil control valve 40may be connected to an AC or DC source of electricity. Both may be madeavailable on the drilling rig.

The energisation and de-energisation of the solenoid of hydraulic oilcontrol valve 40 creates a pulsed supply of hydraulic oil throughhydraulic oil supply duct 44 to chamber 35 of pump 30 determining therate of supply of lubricating fluid to the drilling apparatus. Operationof the solenoid also controls venting of hydraulic oil through breather38 and line 39.

An operator of the drill rig and lubrication system 10 may set theduration and frequency of hydraulic oil pulses to the pump 30 throughhydraulic oil control valve or timer 40. These settings determine thefrequency and rate of lubricating fluid supply, in pulses, to thedrilling apparatus. Frequency and rate of lubricating fluid supplydepends on the nature of the drilling apparatus. Larger diameter hammersgenerally have higher lubricating fluid requirements than smallerdiameter hammers of the same design. Noting this, the capacity of tank20 can also be selected with reference to size of the drillingapparatus. As above noted, and for purposes of illustration:

-   -   tank 20 capacity is 30 litres.    -   pump 30 may deliver a flow rate of between 0 and 3 litres per        minute of lubricating fluid with 375 ml per delivery stroke at        510 psi    -   lubricating fluid (here a lubricant) is delivered at a supply        pressure of 700 psi/50 bar.

Once set, these parameters are typically left unchanged or constant fora given drilling operation.

Lubricating fluid is delivered to chamber 32 through a lubricating fluidduct 51 extending from an outlet 24 of tank 20 to the lubricating fluidport 33 of pump 30. Duct 51 includes a check or non-return valve 55 witha crack pressure of 1 psi (allowing ready passage of lubricating fluidtowards chamber 32 on an intake or suction stroke of pump 30 but nopassage of lubricating fluid back into tank 20 on a delivery stroke), astrainer 53 for filtering the lubricating fluid and a ball valve 27enabling the duct 51 to be closed if required for servicing or otheroperating reasons. Duct 51 is of type 316 stainless steel for corrosionresistance.

Referring to FIGS. 1 and 2, lubricating fluid is delivered throughlubricating fluid port 33 through duct 61 which includes a check valve62, with crack pressure 1 psi (and so in open state on a lubricatingfluid delivery stroke as opposed to closed state of check valve 55 atthe same time) and pressure gauge 63 (maximum rated pressure 60 Bar) formonitoring lubricating fluid pressure. If lubricating fluid pressure ishigher or lower than required, the drill rig operator can vary the abovedescribed settings for the hydraulic oil control valve 40 to address theproblem.

Downstream of pressure gauge 63, lubricating fluid delivery duct 65communicates with an air line 190 between compressor 195 and downstreamportion of the compressed air circuit for DTH hammer 100 as abovediscussed with reference to FIGS. 7 to 10. This ensures entrainment oflubricating fluid in the compressed air and so the compressed aircircuit also forms the lubrication circuit for the DTH hammer 100.Lubricating fluid then passes, through the above described compressedair circuit, to DTH hammer 100 and its working parts (including checkvalve 155 for lubrication, working chamber 144 for lubrication andforming an oil seal; and drill bit/drive sub assembly 125) as abovedescribed. As such lubricating fluid passes through the plurality ofspaces 136 left between the splines 128 and 131, a film of lubricant isformed and maintained because the channels formed by spaces 136 enableworking fluid with entrained lubricating fluid to pass through them andcontact the surfaces of complementary interlocking spline portions128,131 of drill bit/drive sub assembly 125. Control over operation ofpump 30, as above described, provides assurance that the requiredlubrication of the working parts of DTH hammer 100 is achieved.Expectation of premature shanking is significantly reduced.

Ducts 61 and 65 are again of type 316 stainless steel for corrosionresistance.

Pump 30, associated hydraulic oil control valve 40 and ductwork areshown in FIGS. 1 to 4 within housing 10 a. These components oflubrication system 10 may be further protected from moisture and dirt bycontaining housing 10 a within a further protective cover (not shown).This cover, of rectangular box shape and metal fabrication, is mountedon metal rods 81 and fixed into position using nuts.

Modifications and variations to the lubrication system 10 for a drillingapparatus as described in this specification may be apparent to theskilled reader of this disclosure. Such modifications and variations aredeemed within the scope of the present invention. For example, thelubrication system may be applied to other percussive drilling apparatusand is not limited only to the embodiment as described above.

1. A lubrication system for a drilling apparatus including a drillstring and a drill bit, said drilling apparatus being driven by aworking fluid circulated through a working fluid circuit, said systemcomprising: a reservoir of a lubricating fluid for supply to a drillingapparatus during a drilling operation; and a pump for supplyinglubricating fluid from said reservoir to said drilling apparatus duringsaid drilling operation, wherein said pump is controlled to supplylubricating fluid from the reservoir at a determined rate to alubrication circuit formed by said working fluid circuit of saiddrilling apparatus.
 2. A lubrication system according to claim 1,wherein said drilling apparatus is operated from a surface locateddrilling rig, said pump and the reservoir also being surface located,preferably on a deck of the drilling rig.
 3. A lubrication systemaccording to claim 2 wherein said working fluid is compressed air.
 4. Alubrication system according to claim 2 wherein said drilling apparatuscomprises an assembly of said drill bit and a drive sub, said drill bitand drive sub being provided with a plurality of respective andcomplementary spline portions formed about their perimeter, said splineportions interlocking to form said assembly with spaces between saidinterlocking spline portions acting as channels through which bothworking fluid and entrained lubricating fluid pass.
 5. A lubricationsystem according to claim 4 wherein said pump delivers lubricating fluidat sufficient rate to form a film on surfaces of the complementaryinterlocking spline portions of said assembly of drill bit and drivesub.
 6. A lubrication system according to claim 2, wherein thelubricating fluid creates an oil seal between a piston reciprocable in aworking chamber within said drill string for driving said drill bit andthe wall of the working chamber.
 7. A lubrication system according toclaim 2, wherein the determined rate of supply of lubricating fluid tosaid working fluid circuit depends on at least one parameter selectedfrom the group consisting of drill string or hammer diameter, workingfluid pressure and formation type.
 8. A lubrication system according toclaim 2 wherein said pump is a hydraulically operated pump, havinghydraulic fluid pressure within the pump controlled by a timer or acontrol valve device for controlling hydraulic fluid pressure within thepump.
 9. A lubrication system according to claim 8 including pressureregulator(s) for reducing pressure of hydraulic fluid from the drill rigto pressure suitable for operating said pump.
 10. A lubrication systemaccording to claim 8 wherein said pump is a piston pump.
 11. Alubrication system according to claim 10, wherein the piston pump drawsa predetermined quantity of lubricating fluid from the reservoir duringintake stroke, and expels the drawn lubricating fluid during a deliverystroke to the drilling apparatus.
 12. A lubrication system according toclaim 11, wherein the piston pump includes a working chamber comprisingtwo variable volume chambers, one variable volume chamber being on adelivery side of the pump, and the other variable volume chamber beingon a pump side of the pump.
 13. A lubrication system according to claim12, wherein the pump side chamber is pressurised by a hydraulic fluidfor operating the pump during delivery stroke.
 14. A lubrication systemaccording to claim 13, wherein length of travel of the piston of thepump is controlled to control rate of intake of hydraulic fluid intohydraulic side chamber and supply of lubricating fluid to the drillingapparatus.
 15. A lubrication system according to claim 14 whereinhydraulic fluid operating the hydraulic pump is the lubricating fluidcontained in the reservoir.
 16. A lubrication system according to claim13 including at least one non-return valve to prevent return oflubricating fluid to the reservoir when the lubricating fluid isintended to be supplied to the drilling apparatus, or the drillingapparatus when supply to the pump is required.
 17. A lubrication systemaccording to claim 1 wherein the drilling apparatus is a rotary drillingapparatus or a percussive drilling apparatus.
 18. A method of drilling aformation with an earth drilling apparatus including a drill string anda drill bit, said drilling apparatus being driven by a working fluidcirculated through a working fluid circuit, said method comprisinglubricating the drilling apparatus during a drilling operation whereinsaid drilling apparatus comprises a lubrication system having areservoir of a lubricating fluid for lubricating the drilling apparatus;and a pump for supplying lubricating fluid from said reservoir to saiddrilling apparatus during said drilling operation, said pump beingcontrolled to supply lubricating fluid from the reservoir at adetermined rate to a lubrication circuit formed by said working fluidcircuit of said drilling apparatus.